The purpose of the proposed research program is to examine the relation between brush border transport processes and overall transepithelial ion movements across in vitro rabbit ileum during normal absorptive states and abnormal secretory states and to define the responses of these brush border mechanisms to humoral, neural, pharmacologic and pathogenic agents. Particular emphasis will be placed on the interaction between chloride and bicarbonate movements which appear to be mediated by a neutral Na-anion transport process located at the brush border. In addition, studies will be carried out to determine the effect of elevated intracellular cyclic-AMP concentrations on the electrical potential profile across rabbit ileum and on the resistances of the mucosal and baso-lateral membranes. Finally, the effects of elevated intracellular cyclic-AMP concentrations on the properties (both ion-selective and dimensions) of the high conductance extracellular shunt pathway across rabbit ileum will be examined. The overall purpose of the proposed program is to determine whether the secretory state evoked by theophylline, cholera toxin or other agents that elevate intracellular cyclic-AMP concentrations is the result of alterations in "normal" transport processes or whether "de novo" transport processes are elicited.